On Thursday, October 31, 2024, Jakub Jelinek <jakub@xxxxxxxxxx> wrote:
On Thu, Oct 31, 2024 at 10:15:03AM +0200, drago01 wrote:
> On Thursday, October 31, 2024, Jakub Jelinek <jakub@xxxxxxxxxx> wrote:
>
> > On Wed, Oct 30, 2024 at 10:46:01PM -0400, Neal Gompa wrote:
> > > I know the idea of moving to -O3 has been briefly mentioned before in
> > > other contexts when we've discussed uplifting the flags, but it looks
> > > like Ubuntu is moving to -O3 for Ubuntu 25.04[1]. Is there a reason
> > > why we shouldn't consider doing the same for Fedora Linux 42?
> >
> > Yes, this is a very bad idea.
> >
> > -O3 significantly increases code size (and the speed up gains aren't really
> > guaranteed), which is highly undesirable when the vast majority of code in
> > the distro isn't performance critical, the I-cache footprint is then more
> > important.
> Isn't instruction cache footprint already part of "performance" i.e if
> performance is improved it shouldn't matter and vice versa, or what am I
> missing?
That is not how compilers work, ...
That's not what I meant sorry. I mean we shouldn't worry about size or cache footprint, in the end what matters is performance. If we compile with -O3 and binaries are faster it doesn't matter if the code size is larger. Likewise if they are just bigger and perform worse or similar to -O2 it's pointless. Or iow cache footprint and binary size are just factors that determine performance, but not goals on their own.
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